This invention relates to optical analyzers and more particularly to an instrument for measuring the absorption coefficient of light absorbing suspensions, or black carbon in aerosol particulate material.
Recent studies have shown that large concentrations of graphitic, or black, carbon particles are found in the atmosphere in both urban and remote locations. These particles are produced in combustion and have a large absorption cross section, on the order of 10 m.sup.2 /g. Their presence affects radiation transfer through the atmosphere, causing visibility degradation and possible changes in the regional or global balance. The size of these effects depends critically on both the concentration of their particles and their single-scattering albedo, which is determined by the relative magnitude of the scattering and absorption coefficients. The scattering coefficient is easily measured by a nephelometer.
However, measurements of the absorption coefficient of these ambient aerosols are difficult, mainly because of the small magnitude of this coefficient: the coefficient typically being in the order of from about 10.sup.-3 to 10.sup.-6 m.sup.-1. An instrument has been developed by the Lawrence Berkeley Laboratory, of the University of California, to determine the coefficient by measuring the attenuation of a light beam transmitted through aerosol particles that are continuously collected from the atmosphere on a suitable filter. This instrument, named an aethalometer (derived from the Greek word meaning "to blacken with soot"), has been described by A. D. A. Hansen, H. J. Rosen, and T. Novakov, in "Real-Time Measurement of the Absorption Coefficient of Aerosol Particles," Applied Optics, Vol. 21, No. 17, Sept. 1, 1982, pp. 3060 et seq.
In general, the above aethalometer uses a cellulose fiber light-transmitting filter which is partially covered by a transparent mask so that air can be drawn through only a small part of the filter on which the particles are collected. The non-collecting portion of the filter covered by the mask is used as a reference. Light from a stabilized lamp passes through a 530-nm bandpass filter and is then directed by a quartz light guide to uniformly illuminate the collecting and reference areas of the cellulose filter. The light transmitted through these two portions of the filter is picked up by two separate optical fibers set into the filter support, giving signal and reference beam intensities proportional to the magnitude of light transmitted through the two portions of the filter. The fibers conduct these beams to two separate silicon detectors whose outputs are coupled to a logarithmic ratiometer. The voltage output of this unit is proportional to the instantaneous optical attenuation due to the absorption of the collected particles. At selected time intervals, the output signal of the ratiometer is digitized and stored in a suitable computer. This result is subtracted from the previous measurement, giving a difference proportional to the average of the absorption coefficient during the averaging time interval.
The averaging time interval is primarily determined by the concentration of the aerosols in the atmosphere, since there must be sufficient particles collected in the filter during the time interval so that there is a readily detectable difference between the beginning and ending coefficient of absorption. The higher the concentration of aerosol particles, the shorter may be the averaging time interval, and vice versa.
Aethalometers as above described have been quite satisfactory when used at ground stations for measuring the absorption coefficient of aerosol particles in the atmosphere. However, they have not proven satisfactory in very rapidly changing environmental conditions such as the study of clouds of smoke rising from ground level fires. In such clouds, the concentration of black carbon particles is usually quite stratified, with the concentration varying widely at different heights from the ground. In order to make a proper study of such clouds, a plane with an on-board aethalometer should cross through the cloud at different levels, with the absorption coefficient being measured at each level. However, such clouds often have a relatively small diameter such that the flight time through the cloud is insufficient for enough particles to be collected in a single pass to enable accurate determinations of the absorption coefficient at that level to be made.